General introduction and summary
by T.J.T.P. van den Berg
The present report is the result of work over the years 2003 and 2004 performed for the European Community in the field of transport safety. In response to the "call for proposals with a view to obtaining grants in the field of transport" (2001/C 202/12) published 18 July 2001, especially paragraph 3.4 on safety of Inland Transport, a consortium named GLARE was formed at the end of 2002 to study questions related to driving safety, more in particular to the danger of glare. Participants were ophthalmological university clinics in Amsterdam, Barcelona, Salzburg and Tübingen, as well as a research lab in Amsterdam. Later, the clinic in Antwerp joined the group. A grant was obtained from the Directorate-General for Energy and Transport of the European Community. For safe driving, eyesight is beyond doubt a major issue, and we propose to improve upon eyesight testing and upon awareness of eyesight problems. Point 6 of annex III of directive 91/439/EEG, gives minimum demands for the visual capacity of applicants for driver licensing. These demands should be based on proper knowledge of their relation to driver safety. Their definition and way of measurement should be unambiguous. Moreover, several visual functions are not included, notably disability glare. It was the intention of the present investigation to further our knowledge with respect to these questions. The efforts of several groups are combined in a European-broad project. One methodological group and 5 ophthalmological groups all over Europe joined forces to assess the potential of glare testing and the prevalence of visual deficits among European drivers in order to gather data needed for European decision making upon driver licensing.
Although this report reflects the joined efforts of many individuals, most of the actual writing was in the hands of: Gerard de Wit (compilation, intro's and summaries of chapters 2&3, sections 2.4, 3.2 and 3.3.2), René van Rijn (chapter 1), Luuk Franssen (sections 2.2, 2.6 and 3.5), Joris Coppens (section 2.3) and Tom van den Berg (sections 2.7, 3.2, 3.3.1 and 3.3.3).
Relations between impaired vision and increased rates of traffic accidents
and violations have been found on many occasions (for a review, see van Rijn and
Völker-Dieben, 1999). It is therefore not surprising that demands are being
placed upon the visual functions of drivers (Council directive 91/439/EEC).
Impairments of visual functions are mainly problems of the elderly driver. Many
studies have shown that visual functions decline with age. This holds for visual
acuity, but also for visual field and other modalities of visual function, such
as glare sensitivity and visual attention. For this reason, testing of the
visual functions is of particular relevance in elderly drivers. In the years
2001-2002 several members of the present group joined in a combined project to
study the technologies available for visual function testing. Important
conclusions were that (1) retinal straylight offers added value to the tests
that already exist and that a practical instrument is needed to assess retinal
straylight in the large population (2) for decision making on what tests to
include in driver licensing regulation, and at what cut off level, Europe wide
prevalence values are needed.
The subsequent GLARE project aimed to give a better hold on the danger of glare in driving. Although the general belief is that glare is a serious thread for driving safety, it has escaped up till now proper evaluation. The reason is that no reliable method was available to determine glare sensitivity in (candidate) drivers. The project started by designing a suitable instrument to measure glare sensitivity (i.e. retinal straylight) for driver licensing application, working from the experience of the earlier collaborative project during the years 2001-2002. At the same time a measurement protocol was set up to determine the percentages of visual impairments among European drivers (the prevalence part of the study). The old visual functions (visual acuity and visual field) as well as new ones (in particular retinal straylight, but also contrast sensitivity, and "Useful Field Of View") were included. Also basic studies to elucidate the relationship between retinal straylight and actual visual handicap during driving took place.
CHAPTER 1. PREVALENCE OF VISUAL FUNCTION IMPAIRMENT.
In this chapter the results are presented from the prevalence part of the study. Insight in the prevalence of vision impairments in the driving population is important, both for evaluating the effectiveness of the current regulations on visual functions of drivers and for estimating the impact of possible new regulations. We investigated 2422 drivers in 5 different countries Netherlands, Spain, Germany, Austria and Belgium in the following age categories: 45-54 years, 55-64 years, 65-74 years and 75- years of age and older. In addition, there was a smaller group with ages between 20 and 30 years, to serve as a reference group. Many visual functions were tested, in particular retinal straylight using a method designed in this same project (Chapter 2). In addition, subjects were asked to fill in two questionnaires. One questionnaire was about driving habits, driving difficulties and self reported accidents. The other questionnaire was the NEI-VFQ25 into vision related quality of life. All subjects underwent an ophthalmological examination, comprising the evaluation of current and past ocular diseases and abnormalities.
Prevalence of impairments of visual functions was found to be low in the younger age groups, and raises to relevant percentages in the higher age groups. This counts for all modalities of visual function, but especially for those functions that are not included in the current regulations, such as straylight and Useful Field of View. The percentage of subjects with inadequate correction of their refractive error is rather high and about equal in all age groups. Quite a number of subjects do not meet the current European standards on visual acuity and visual field, particularly in the highest age groups. The requirements on visual acuity could be met in the majority of cases if refractive errors were adequately corrected. In all age groups, acuity can be improved in a significant number of subjects by optimization of correction of refractive errors, although in the younger groups, the majority of subjects, even with their (sub-optimal) habitual driving correction, still meet the current standards.
Our findings regarding the higher prevalence of impairments of contrast sensitivity and stray light in the elderly groups are in concordance with the larger number of eye diseases and abnormalities that are found in these age groups. Past research has demonstrated the importance of adequate contrast sensitivity for driving safety. This suggests, in combination with our results, that contrast sensitivity could have a more important role in the assessment of drivers than in the current regulations. For such role, a better establishment of cut-off values would be necessary. Past research into the role of stray light in traffic safety has been hampered by the lack of an adequate measurement method. Our results demonstrate that stray light sensitivity can be adequately measured in the majority of subjects in a population study, facilitating future research into its relevance.
CHAPTER 2. STRAYLIGHT MEASUREMENT TECHNIQUE.
Glare sensitivity is considered an important parameter for traffic safety. Studies by Von Hebenstreit and Lachenmayr show a relatively strong relationship between glare sensitivity and road traffic accidents. Basically what happens in a glare situation is that light from the glare source (e.g. headlights) does not only follow geometric optical rules to form an image of the glare source on the retina, but also some of the light is scattered in the eye creating a veil of light on top of the image of the rest of the visual field. This light is called retinal straylight. So a technique is needed to determine an individual's retinal straylight level. In fact, in the earlier study (van Rijn et al. 2005) several conventional glare tests and straylight meters were evaluated for their ability to discriminate clinically evaluated cataract from non-cataract patients. This study showed that a straylight meter is better in several aspects such as discriminative ability than the conventional glare tests.
In the current study the straylight meter was further developed to make it suitable for large scale application such as in the prevalence study of chapter 1 and for driver licensing testing. The new method is called "compensation comparison" method. Central is the application of the so called 2 alternatives forced choice method (2AFC method). This method is well known from psychophysics for its reliability. Reproducibility that was finally achieved during the study is around 0.07 log units for the majority of individuals, and 0.1 log units overall. See figure 2.19. Furthermore, the task has become more intuitive and comfortable for the subject. An important consequence has been that the German firm OCULUS, well established in ophthalmology has chosen this approach to design a straylight meter for the commercial market, called C-Quant.
CHAPTER 3. RELATION STRAYLIGHT - DRIVING SAFETY
For straylight testing in driver licensing, ultimately we would like to understand the relationship between a certain straylight outcome and road safety. This relationship involves a very complex set of intermediate steps and separate issues. In the present chapter we set out to contribute on a few points to this understanding. From simple to more complex, and more close to the actual driving situation, the following 3 steps were approached: The simple static blinding situation. Straylight induces sensitivity loss and blinding, depending on the relationship between visual task and glare source. Especially important are the luminance relationships. In this chapter we will review the luminances encountered during night driving and the glare induced blinding (equivalent) luminances. The simple dynamic blinding situation. Driving is a dynamic process. Also blinding is a dynamic process. Glare sources are often encountered suddenly while driving, and for a short period. Blinding is more strong if the blinding source is presented in such a dynamic fashion. Experimentally this increment was determined, and proved to be around a factor of 2. The complex blinding effects during driving. Although ideally steps 1 and 2 could predict what drivers may or may not be able to see, one would like this to be demonstrated in actual road tests. Because of several practical, legal and ethical reasons this might better be studied with simulation. We entered in to a largely FDA funded study using the NADS system in Iowa. The first pilot experiments have been done, but not the real test. We will continue to participate after closure of the present European project, and the data will become available also for Europe.
In this chapter also the search for realistic (early) cataract simulation filters is reported. The importance of such filters is for education/demonstration, to improve public awareness, to show policy makers what driving looks like with increased straylight, etc. We also use these filters to increase straylight in a realistic manner in experimental subjects performing the visual- and driving performance tests.